1. Field of the Invention
The present invention relates generally to broadcasting visible light information, and in particular, to an apparatus and method for broadcasting visible light information using multiplexing.
2. Description of the Related Art
The Ministry of Commerce, Industry and Energy (MOCIE), forecasts that Light Emitting Diodes (LED) will replace other existing lighting devices by 2015. As LEDs improve in light emitting efficiency and decrease in price, LEDs are becoming more popular not only in the special lighting market for portable devices, displays, automobiles, traffic lights, advertising electric bulletin boards, etc., but also in the normal lighting market for fluorescent lamps, incandescent lights, etc. In particular, the white LEDs have already reached parity with incandescent lamps in terms of the light emitting efficiency, and LED products superior even to fluorescent lighting are also available now.
Recently, study of wireless visible light communication based on visible LEDs, which are being conducted in many enterprises and research institutes as the interests in wireless optical technologies and are complementary to Radio Frequency (RF) technologies, has been increasing due to the exhaustion of RF frequencies, the possible interferences between several wireless communication technologies, the required increase in communication security, and the advent of the ultra-high-speed ubiquitous communication environment of the 4th Generation (4G) wireless technology.
Lighting lamps, such as fluorescent lamps and incandescent lights now used in households, offices and public places, will be replaced in the near future by LEDs having high performance and high durability. Lighting LEDs can also be used as light sources for communication by modulating a current being applied to the LEDs that are used as lighting lamps. In other words, it is possible to transmit/broadcast data only with lighting LEDs, without additional light sources.
Delivering information using visible lights is advantageous in that visible light communication can provide visibility by which the communication link can be checked by the naked eye and can also guarantee reliable security. Visible light communication also has various uses and, in contrast with radio communication, visible light communication can be freely used without regulations. Visible light communication can also simultaneously perform lighting and communication functions. That is, in visible light communication, normal lighting equipment can transmit and receive information as a visible light communication transceiver, while simultaneously serving as a lighting source.
Visible light communication devices based on LED pixels adopt, as a communication scheme for displaying data, a communication scheme that transmits/receives data to/from each other using three colors of Red, Green, and Blue (RGB). This visible light communication system performs data communication through instantaneous switching or adjusting of the visible lights.
FIGS. 1A and 1B are graphs illustrating data signals which are commonly modulated according to time and wavelength in visible light communication. FIG. 1A illustrates data signals obtained by simultaneously modulating output signals of three RGB LEDs, while FIG. 1B shows data signals created by individually modulating output signals of three RGB LEDs.
Referring to FIG. 1A, one modulated data signal is obtained by simultaneously modulating output signals of three RGB LEDs with the passage of time, and the modulated data signal corresponds to any one of a case where the three LEDs are all turned on and another case where the three LEDs are all turned off. The former case where the three LEDs are all turned on is suitable for long-distance communication where the modulated data signal can be received even at the location of a far-away user (for example, a user located far away from the LED display), as the signal is output in white. However, it is not possible to simultaneously transmit many data signals since only one data signal can be modulated at a time.
Next, referring to FIG. 1B, data communication is performed by modulating output signals of three RGB LEDs individually with the passage of time. When the output signals of three RGB LEDs are modulated individually, data transmission is possible using a combination of the RGB signals. However, the individual modulation of FIG. 1B can be used for short-distance communication where the data can be received by a nearby user (for example, a user located near the LED display), as the output power of the LED is low in comparison to the simultaneous modulation of FIG. 1A.
Accordingly, there is a demand for a method of transmitting necessary data by changing a modulation method for output signals of three RGB LEDs at stated intervals to satisfy both the far user and the near user.